Loop extenders (such as the one described in U.S. Pat. No. 3,763,320 which issued on Oct. 2, 1973 to C. W. Chambers) are used to increase the signalling range for originating the terminating calls on relatively long subscriber lines by adding a series-aiding boost voltage to either or both sides of the line, thus effectively increasing the loop supply voltage. Subscriber loop facilities of different types requiring such treatment, however, have various different operating conditions that must be accommodated to ensure satisfactory operation.